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Two decades ago, Frank Speizer, one of the Harvard Six Cities Study investigators, showed me a startling finding of a positive association between the concentration of particulate matter in outdoor air and all-cause mortality.1 At the time, I expressed caution and offered confounding as a potential explanation; he countered by showing me the initial analyses of air pollution and mortality in the American Cancer Society’s Cancer Prevention Study II (CPS-II), which replicated the association.2 Particulate air pollution had previously been associated with longer-term mortality, but such an association had been considered implausible at the lower ambient concentrations present by the 1980s, as air quality had improved.3–6 Beginning in the late 1980s, however, daily time-series studies linked short-term variation in mortality to various air pollutants, particularly particulate matter. Whether there was substantial life-shortening from air pollution remained controversial.

The findings of the two cohort studies were particularly significant because they provided evidence that air pollution resulted in more than a brief displacement of the time of death of already ill people (a phenomenon initially referred to by the unfortunate term of “harvesting” and later described as “mortality displacement”). In 1997, the US Environmental Protection Agency implemented a new standard for fine particulate matter (particulate matter less than 2.5 microns in aerodynamic diameter or PM2.5) (http://www.epa.gov/ttn/naaqs/standards/pm/s_pm_index.html). The foundation of evidence underlying the new standard included the positive associations found in the daily time-series studies and the findings of the Six Cities Study and CPS-II, together indicating that particulate matter air pollution was still a problem with substantial public health impact.7 The emphasis given to the two cohort studies by the Agency led to calls for access to their data and to an independent reanalysis that replicated the findings of the original investigators and found them to be robust to refinements of analytic methods and tighter control of confounding than in the original analyses.8 Subsequently, air pollution has been linked to increased mortality in other cohort studies.9–11

Tremendous gains have been made in air quality in the United States since the Clean Air Act was passed in 1970 (Figure). These gains have led to calls for documentation of the public health benefits of improved air quality: Has the risk associated with air pollution dropped as levels have declined? Are there parallel trends of a gain in life expectancy and of a decline in morbidity? So-called “accountability research” has been directed at these questions, comparing observed effects of air pollution with those projected as counterfactuals under scenarios of unchanged air quality.12 While such research has proved challenging,13 an analysis based in 51 metropolitan areas included in CPS-II showed an association between decline in fine particulate air pollution and gain in life expectancy over the two decades of the 1980s and 1990s.14 A decline of 10 µg/m3 in annual PM2.5 concentration was associated with a mean life expectancy gain of about 0.6 years.14

NEW FINDINGS OF CORREIA ET AL

In this issue of EPIDEMIOLOGY, Correia et al15 present analyses that expand and update the earlier work of Pope et al14 on decline in air pollution and gain in life expectancy. Since the promulgation of the PM2.5 National Ambient Air Quality Standard, monitoring for this pollutant has been implemented nationally. Correia et al15 use the resulting national data on PM2.5 for 2000–2007 to test whether declines in this pollutant over the 8 years have been accompanied by a parallel gain in life expectancy. A total of 545 counties had the requisite air pollution data; the estimated gain in life expectancy over the study period was 0.35 years per 10 µg/m3 decline in annual PM2.5 concentration—lower than estimated by Pope et al14 for the earlier time period and a different set of counties. The association was robust to adjustment for potential confounding by changes in sociodemographic variable and smoking. The authors also extended the analysis of Pope et al14 to the more recent time period, generally replicating the earlier findings. In the geographic locations included in the earlier study of Pope et al14, the estimated gain in life expectancy during 2000–2007 was substantially greater than during the earlier period, even though concentrations were lower.

Several other overall findings of Correia et al15 merit emphasis. The gain was greater in urban areas and among women. The gain did not vary by level of PM2.5, implying that additional gains in life expectancy would follow a further decline in the concentration of this pollutant, regardless of the starting concentration.

The authors acknowledge the key limitations of their analysis, which reflect the inherent limitations of the models used and the input data available. During the period 2000–2007, cardiovascular mortality continued to decline16—a decline driven by therapeutic advances and declining incidence—as did mortality from smoking-caused cancers.17 The authors gave consideration to a dominant risk factor—smoking—but had only a surrogate indicator, and the available data did not cover other factors that might have contributed to rising life expectancy. An informative extension of their analyses would address mortality from cardiovascular disease (the leading cause of death), and from smoking-caused cancers.

IMPLICATIONS

Setting aside limitations and particularly the possibility of uncontrolled temporal confounding, the results of Correia et al15 have potential implications for air quality regulation, as they suggest that further reduction of today’s air pollution levels would benefit public health. In the United States, particulate matter air pollution is regulated under Sections 108 and 109 of the Clean Air Act. For particulate matter and five other key pollutants, Section 109 of the Act requires the Administrator of the Environmental Protection Agency to set a national air quality standard that protects the public health with an “adequate margin of safety.” This margin acknowledges uncertainty and the need to protect the more susceptible population groups.

For both particulate matter and ozone, recent evidence indicates that adverse effects are associated with the current low concentrations of these pollutants, in spite of the gains since the Clean Air Act was passed in 1970 (Figure). This evidence has complicated any revisions of the national standards, making it difficult to identify a concentration that provides the requisite margin of safety. In considering revisions to the national air-quality standards, the Agency now carries out risk assessments that estimate the burden of disease at current ambient concentrations, as well as the burden that would be avoided by promulgation of a stricter standard than the existing. In these analyses, scenarios are considered that take into account the current standards, the contributions of controllable and natural sources to pollutant concentrations, and attainability—although cost cannot directly figure into the Administrator’s decision.

The findings of Correia and colleagues15 add to the evidence suggesting that such scenarios should be extended downward toward the lowest achievable concentrations for airborne particulate matter. The estimated benefits for life expectancy would be an important consideration for the Administrator of the Environmental Protection Agency in promulgating national standards under the public health mandate of Section 109. The methodology used by Correia et al15 is straightforward, and their study could be readily repeated to assess whether future rounds of revision in national air-quality standards lead to increased life expectancy.

The findings also point to the role of air pollution more globally in contributing to the burden of avoidable premature mortality.18 Much of the world’s population remains exposed to particulate matter air pollution at concentrations one or two orders of magnitude higher than those in the United States during the years of 2000–2007, as considered by Correia et al.15 Exposure levels in many countries reflect high levels of outdoor air pollution from vehicles, industry and biomass burning, and indoor air pollution from biomass fuels and tobacco smoking. Major initiatives are finally underway to reduce indoor exposures to smoke from biomass combustion, but outdoor air pollution receives little regulatory attention in many countries. Control of outdoor air pollution must not be neglected as economic development drives up pollution emissions. The findings of Correia et al15 suggest that air pollution control in such settings could extend life expectancy substantially.

15. Correia AW, Pope CA, Dockery DW, Wang Y, Ezzati M, Dominici F. The effect of air pollution control on life expectancy in the United States: an analysis of 545 US counties for the period 2000 to 2007. Epidemiology. 2013;24:23–31